When the spoken word won't do, put pen to paper. Fingers to a keyboard works fine as well. Letters to teenagers may be advisory, congratulatory, explanatory or merely friendly, but they must have the one quality that teenagers are sensitive to, honesty. Today's teens aren't of a generation that required letters to communicate, so at the very least, they will appreciate your effort for its novelty. The best advice to take in this situation is that which is dished out to teenagers on a daily basis: just be yourself.
Choose between a handwritten letter or a typed one. The decision should be based on how the letter will be sent, how quickly it must get to the recipient and which medium will be more appealing to the teenager you are writing to. A heartfelt letter may be better expressed through the earnestness of longhand.
Use all of letter writing conventions that you were taught in school. They go a long way to communicate respect for the recipient, whatever her age. Open with her proper title.
Start the body of the letter with a warm greeting and a question or two that show your interest in the teen you are writing to. For example, ask him if he's been to a good concert or film lately or what his post-graduation plans are.
Keep the tone of the letter open and casual.
Use the same "voice" and words as you would if you were talking to this teen in person. She should be able to hear you saying the words of the letter as she is reading them.
End the letter with a gracious valediction, such as "Best regards".
Avoid giving unsolicited advice. It may come off as being pretentious.
Do not use slang or language that you wouldn't otherwise use. It will look forced.
Avoid questions that pry into personal matters or emotional subjects.